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A new study helps to explain what separates the fleet-footed from the heavy-footed: ankle structure and toe length.

While these aren't the only features that make sprinters fast, subtleties in anatomy might help coaches determine whether to steer young runners towards distance or short races.

The insights could also lead to faster sprinting shoes and even help people stay mobile into old age.

"Track coaches always say that sprinters are born not made," says Associate Professor Stephen Piazza, a biomechanist at Pennsylvania State University. "We can't answer that question with a study like ours, but it sounds like something that is set by genetics."

Piazza's study had serendipitous beginnings. He and colleagues were working with a football player at Penn State University who wanted to prolong his career with a scientific approach.

Among a battery of tests, Piazza used ultrasound to measure something called the lever arm of the athlete's Achilles tendon.

When someone runs or walks, the calf muscles pull up on the Achilles tendon at the back of the heel. Where the muscle attaches to the tendon varies from person to person, and the length of this connection works as a lever, sort of like a seesaw.

Surprise finding

The longer the lever arm is, the more leverage it has. It's the same concept that explains why it's easier to push up on a seesaw if you're sitting further away from the plank's central pivot point.

Since the football player was a super-speedy wide receiver who went on to play in the NFL, Piazza expected him to have the longest lever arm he had ever measured. Instead, measurements revealed the opposite.

"We thought maybe he had an injury, but the other leg was the same," says Piazza. "We thought maybe something was wrong with the equipment," but measurements on other people came out as expected.

To follow up, Piazza and colleague Sabrina Lee used ultrasound measurements to compare the ankles and feet of 12 competitive college sprinters and long jumpers with those of 12 graduate students who were not sprinters but whose heights were similar.

Measurements revealed that the sprinters had Achilles lever arms that were 25% shorter than the lever arms of the non-sprinters.

While surprising, the findings actually make sense, says Piazza. With smaller Achilles lever arms, the calf muscles have less leverage but more force because they don't have to shorten as much or as quickly with every stride.

Longer toes provide more contact with the ground for a more effective push-off, he says.

"Early in the race is where it's won or lost," Piazza says. "If you can maintain contact with the ground longer, you can get more speed earlier."

Computer simulations confirmed the ultrasound findings. So did a look at the animal world.

Cheetahs, for example, have shorter Achilles lever arms and longer toes than lions do. And speedy runners have shorter lever arms than do animals that dig, swim or stroll.

Other factors

Ankle structure certainly isn't the only factor that gives sprinters their edge, nor is it the dominant one. A person's sense of coordination, reaction times and proportion of fast-twitch muscle fibres also make a difference.

"It's the combination of every little factor that determines whether you are a winner or not," says biomechanist Mario Lafortune, Director of the Nike Sport Research Lab. "It's a very important piece in the puzzle of understanding speed and distance running."

Piazza is interested in using his research to help people continue to walk normally as they age. Exercises that help older people behave more like sprinters, his newest work is showing, could make a big difference.

"Coming up with ways to help people run one-tenth of a second faster is not all that satisfying to me personally," says Piazza. "When you lose the ability to walk normally, that has a huge effect on your quality of life."